Crystal filter with temperature compensation

ABSTRACT

A narrow band transmission system using a crystal filter having a pass band width in the order of one hertz and operable over an ambient temperature range of 60* C. The output of a crystal controlled oscillator is mixed with the incoming signal to provide a sum signal as the input to the filter. The oscillator and filter crystals have matched temperature characteristics so that the oscillator frequency and the filter center frequency vary in the same amount with change in temperature.

llted States Patent 1 Arnold et a1.

[ June 12, 1973 CRYSTAL FILTER WITII TEMPERATURE COMPENSATION [76]Inventors; James G. Arnold, 1624 South Augusta Place; Lloyd 1). Myers,8401 East Hayne Place, both of Tucson, Ariz. 85710 [22] Filed: July 6,1971 [21] Appl. No.: 159,975

[52] US. Cl 325/416, 325/489, 333/72 [51] Int. Cl. 1103b 3/04, H04b 1/16[58] Field of Search 325/416, 489;

[56] References Cited UNITED STATES PATENTS 2,070,732 2/1937 Holden325/489 2,266,658 12/1941 Robinson 333/72 Primary Examiner--I-Ioward W.Britton Attorney-Harris, Kern, Wallen & Tinsley [57] ABSTRACT A narrowband transmission system using a crystal filter having a pass band widthin the order of one hertz and operable over an ambient temperature rangeof 60 C. The output of a crystal controlled oscillator is mixed with theincoming signal to provide a sum signal as the input to the filter. Theoscillator and filter crystals have matched temperature characteristicsso that the oscillator frequency and the filter center frequency vary inthe same amount with change in temperature.

5 Claims, 2 Drawing Figures FILTER FILTER CRYSTHL OSCILLHTOR QRYSTALFILTER WITH TEMPERATURE COMPENSATION This invention relates to narrowband transmission systems and in particular, to a new and improvednarrow band system utilizing a crystal filter with a pass band in theorder of one hertz, with the system being operable over a wide range ofambient temperature while maintaining the desired center frequency forthe pass band.

Data transmission systems are often utilized where it is desirable toaccommodate very low data rates or where, because of the low frequencyresponse of the transmitted data, it is desirable to maintain verynarrow filter bandwidth. In such systems, the extremely narrow bandwidthprovides maximum noise discrimination and/or data frequency selectivity.A bandwidth in the order of a few hertz or lower is readily obtainedwith a conventional crystal filter. However, drift due to temperaturechange produces shifts in the center frequency which may readily exceedthe bandwidth and which results in rejection of the transmitted datafrequency.

A conventional crystal filter operating at approximately khz can have abandwidth of approximately 1 hz. However, with a temperature variationof 60 C., the resonant frequency of such a crystal varies by about 100ppm or by about 2.25 hz. Such a drift in center frequency for the passband is not compatible with a l hz bandwidth. In one solution for thisproblem, the temperature of the crystal is maintained substantiallyconstant by installing the circuitry in an oven with suitabletemperature stabilization. However, the solution is not always asatisfactory one because of the space required and/or the powerconsumption.

It is an object of the present invention to provide a new and improvedtemperature compensation for a narrow band transmission system utilizinga crystal filter. A further object is to provide such a systemparticularly suited for use with a pass band in the order of one hertzand which does not require a temperature control for the filter crystal.Other objects, advantages, features and results will more fully appearin the course of the following description.

In the drawing,

FIG. 1 illustrates a narrow band transmission system incorporating apreferred embodiment of the present invention; and

FIG. 2 illustrates a narrow band transmission system having two filtersin cascade and incorporating the present invention.

The input signal of interestfl is connected via line 10 as one input toa mixer 11. Typically the input signal), may be the intermediatefrequency from a radio receiver 12.

A second inputf for the mixer 11 is provided via line 13 from a crystalcontrolled oscillator 14. The mixer operates in the conventional mannerto provide an output on line 17 of a frequency which is the sum of thetwo input frequencies.

The mixer output is amplified in an amplifier 18 and connected as aninput to a crystal filter 19, with the filter output connected to adetector 20, to provide the information at output line 21.

The filter 19 is a conventional crystal filter which has a narrow passband, with a bandwidth of l hertz in the example illustrated. The centerfrequency of the pass band of the filter is f, and, in the embodimentillustrated, an incoming signal f,- having a frequency equal to f f i hzwill be passed through the filter to the detector and all other incomingsignals will be rejected.

Compensation for change in ambient temperature is accomplished byutilizing matched crystals in the oscillator l4 and the filter 19. Thetwo crystals will be operating at resonant frequencies of the same orderof magnitude and by matching the drift characteristics, the change infrequency f and in frequency f with change in temperature over a rangeof 60 C will be substantially the same, so that the frequencyselectivity characteristic of the system is maintained. Preferably bothcrystals are similar cuts from identical material. Desirably thefrequencies f, and f are selected so that the resonant frequencies ofthe crystals differ by not more than 20 percent and preferably by notmore than l0 percent.

In a typical example, frequency f may be 22.5 khz and frequencyf may be20 khz. The system is then selective for an input frequencyfl of 2.5 khzi /z hz. The range of the frequencies f f is determined mainly by theband width desired and by the Q of the available crystals. For abandwidth of 1 M with a single crystal, frequencies of 10 hz to 40 hzare useful.

In the circuit of FIG. 2, components corresponding to those of FIG. 1are identified by the same reference numerals. Two single crystal filtersections 19a and 1912 are connected in cascade with an intermediateamplifier 18a to provide a higher order filter with the l hertzbandwidth and a skirt rejection of l2db/octave, rather than the6db/octave of the single crystal filter.

The filters 19a and 19b are tuned to center frequencies 0.353hz aboveand below the desired center frequency f with each having a bandwidth of0.707hz. The crystals of the oscillator 14 and filters 19a and 1% areselected to be matched, as in the circuit of FIG. 1. Three or morefilter sections may be cascaded in the same manner to provide higherorder temperature compensated filters.

The narrow band system of the invention may be used wherever a narrowpass band is desired in an electrical signal handling system. By way ofexample, the narrow band system may be used at a radio receiver toextract or pass a narrow band signal which has been superimposed on avoice channel of a radio transmitter.

Thus it is seen that the frequency selectivity of a very narrow bandsystem can be precisely maintained over a wide variation in ambienttemperature without requiring actual temperature control and itsattendant size and power requirements. Although an exemplary embodimentof the invention has been disclosed and discussed, it will be understoodthat other applications of the invention are possible and that theembodiment disclosed may be subjected to various changes, modificationsand substitutions without necessarily departing from the spirit of theinvention.

We claim:

1. In a narrow band transmission system, the combination of:

an oscillator for producing an oscillator output of a first frequency,and having a first crystal for frequency control;

a mixer;

means for connecting an input signal of a third frequency and saidoscillator output to said mixer as inputs for producing a mixer outputwhich is acombination of said first and third frequencies;

a first narrow band filter having a second crystal for defining the passband of said first filter at a second frequency with a band width in theorder of l hertz; and

means for connecting said mixer output as an input to said first filter;

with said first and second crystals having matched temperaturecharacteristics so that said oscillator first frequency and said filterpass band second frequency vary in the same amount with change intemperature.

2. A system as defined in claim 1 in which said second frequency isequal to the sum of said first and third frequencies.

3. A system as defined in claim 1 in which said third frequency is anintermediate frequency from a radio receiver and in the order of severalthousand hertz, and said first and second frequencies are of the sameorder of magnitude and greater than said third frequency.

said second and fourth frequencies varies in opposite directions fromthe sum of said first and third frequencies a small deviation in theorder of a hertz or less.

1. In a narrow band transmission system, the combination of: anoscillator for producing an oscillator output of a first frequency, andhaving a first crystal for frequency control; a mixer; means forconnecting an input signal of a third frequency and said oscillatoroutput to said mixer as inputs for producing a mixer output which is acombination of said first and third frequencies; a first narrow bandfilter having a second crystal for defining the pass band of said firstfilter at a second frequency with a band width in the order of 1 hertz;and means for connecting said mixer output as an input to said firstfilter; with said first and second crystals having matched temperaturecharacteristics so that said oscillator first frequency and said filterpass band second frequency vary in the same amount with change intemperature.
 2. A system as defined in claim 1 in which said secondfrequency is equal to the sum of said first and third frequencies.
 3. Asystem as defined in claim 1 in which said third frequency is anintermediate frequency from a radio receiver and in the order of severalthousand hertz, and said first and second frequencies are of the sameorder of magnitude and greater than said third frequency.
 4. A system asdefined in claim 1 including: a second narrow band filter having a thirdcrystal for defining the pass band of said second filter at a fourthfrequency with a bandwidth in the order of 1 hertz; and means forconnecting the output of said first filter as the input to said secondfilter; with said first, second and third crystals having matchedtemperature characteristics so that said first, second and fourthfrequencies vary in the same amount with change in temperature.
 5. Asystem as defined in claim 4 in which each of said second and fourthfrequencies varies in opposite directions from the sum of said first andthird frequencies a small deviation in the order of a hertz or less.